1. Field of Invention
The present invention relates to output stages, and more particularly to common mode control loops for output stages.
2. Description of Related Art
The present invention relates generally to low power equipment such as modem telecommunications systems, cell phones, displays, and simple battery powered devices. Electronic equipment typically contains operational amplifiers, for which low power consumption and good dynamic performance are important design considerations. These designs goals require efficient techniques for bias control.
Supply voltages in electronic equipment are continually decreasing. Even high-speed components in common signal chains are required to use the lowest practical voltage to minimize power consumption. However, modem standards for video and high-accuracy data-acquisition systems require large signal swing capabilities to optimize dynamic performance, including voltage swings of 2V or greater. This makes it important for analog circuitry to make use of increasing portions of the supply voltage range. Examples of such analog circuitry include rail-to-rail gain stages, in which the output voltages can swing almost from the voltage of one supply rail to the voltage of the other. Such an operation of a gain stage is referred to as “rail-to-rail” operation. Rail-to-rail amplifiers often use a class AB architecture, sometimes referred to as push-pull architecture.
A problem with building amplifiers with rail-to-rail gain stages, however, is that they tend to be more complex than traditional amplifiers, thus requiring a high number of circuit elements.
In U.S. Pat. No. 5,162,751, entitled “Amplifier Arrangement”, to Blanken and Thus, hereby incorporated by reference in its entirety, a low voltage class AB amplifier arrangement is described. Aspects of this amplifier design include a class AB gain stage, in which the signal and the bias current control share the same path. In such amplifiers the dynamic performance of the signal path is the same as the bias current control path, and vice versa. This directly couples the effects of one to the other, limiting the optimization of the design for good dynamic response of the signal. Furthermore, the feedback circuitry of the described design operates in a voltage mode, whereby modular design changes, for example changing Bipolar to CMOS output transistors, is difficult to achieve. Finally, the problems addressed by this design include operating at low voltages, such as at 1.2V, possibly at lower frequencies, such as 1 MHz.
U.S. Pat. Nos. 5,521,553, 5,440,273, 5,786,731, 5,162,751, 4,335,358, and 5,734,296, all hereby incorporated by reference in their entirety, describe a variety of related efforts for improved performance of gain stages and amplifier circuitry. Yet there still remains room for improvement concerning minimizing complexity, reducing component count, achieving stable operation, and improving dynamic performance.